Turbine blade measurement

ABSTRACT

An invention is disclosed for positioning an object wherein three point supports provide three contact points which define a first plane. Three additional point supports provide three additional contact points which are not coplanar with the first plane and which define a second plane. The first and second planes are nonperpendicular. The point supports accurately and repeatably position the object for optical measurement of a dimension of the object. The invention can be used in connection with the quality control testing of weldments which attach blade tips to gas turbine engine blades.

The invention herein described was made in the course of or under acontract or subcontract thereunder (or grant) with the Department of theNavy.

The present invention relates to fixturing devices and, moreparticularly, to such devices which position an object in space withrepeatably high accuracy for measurement purposes.

BACKGROUND OF THE INVENTION

One method of ascertaining the quality of a diffusion-bonded weldmentrequires the measurement of the weldment's thickness. This measurementcan be undertaken as shown in FIG. 1, which illustrates a caliper 2having anvils 4 and 6 measuring the total thickness of two bondedcomponents 8 and 10 together with the weldment 12 contained betweenthem. Comparison of this total measured thickness with the measuredtotal thickness of the two components without the weldment, prior tobonding, will allow a computation of the thickness of the weldment bysubtraction. As will now be discussed, accurate measurement requiresaccurate positioning of the anvils 4 and 6 with respect to thecomponents 8 and 10. Such positioning is commonly called fixturing.

If one of the components has a rough surface, such as the surface of ametallic casting taken directly from a sand mold, problems arise in thethickness-measuring procedure. One problem is illustrated in FIG. 2,which shows two possible positions of the anvil 6 of FIG. 1, but inenlarged form and labeled as 6A and 6B. The surface 9 of the component10 is rough and, as the alternate positions 6A and 6B of the anvil 6indicate, the measured thickness of the components 8 and 10 in FIG. 1will depend upon the anvil position chosen. Thus, the measured thicknesshas no absolute definition.

Further, even if no absolute measurement is sought, but only a relativecomparison of two successive measurements, problems arise. Accuratecomparison requires that the anvils 4 and 6 be positioned exactly in thesame position, such as the position 6B for anvil 6, for bothmeasurements. This is difficult to achieve.

Still further, it is possible that the components 8 and 10 may becomenicked or scratched between measurements. If a scratch occurs at thelocation where the anvil 6 contacts the component, random error isintroduced. All of these problems are worsened when the components 8 and10 are of irregular shapes, as are sand-cast gas turbine engine blades.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide a new and improvedfixturing device.

It is a further object of the present invention to provide a new andimproved fixturing device for accurately and repeatably positioning anobject in space.

It is a further object of the present invention to provide a new andimproved fixturing device for positioning an object therein forascertaining the integrity of a weldment through measurement of theweldment's thickness.

It is a further object of the present invention to provide a new andimproved fixturing device which does not use flat surfaces to contactthe object to be fixtured to thereby reduce the effects which nicks andscratches on the object otherwise have upon the positioning of the flatsurfaces.

It is a further object of the present invention to provide a new andimproved fixturing device for positioning a gas turbine engine blade inspace for ascertaining the integrity of a weldment therein bymeasurement of the weldment's thickness.

SUMMARY OF THE INVENTION

In one form of the present invention, six point supports are fastened toa bracket at six predetermined positions to support an object. Three ofthe point supports define a first plane, and the remaining three definea second plane. The remaining three must not be coplanar with the firstthree point supports, and the first and second planes must benonperpendicular.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates two components and a weldment between them beingmeasured in thickness.

FIG. 2 illustrates an enlarged view of part of FIG. 1.

FIG. 3 illustrates an object in a coordinate system.

FIGS. 4, 5, and 6 illustrate various principles of support of the objectof FIG. 3 utilized by the present invention.

FIG. 7 illustrates one form of the present invention supporting aturbine engine blade.

FIG. 8 illustrates part of the apparatus of FIG. 7.

FIG. 9 illustrates reference directions of a turbine engine blade.

FIG. 10 illustrates in greater detail the support of the blade in FIG.7.

FIG. 11 illustrates in greater detail a feeler pin of the apparatus ofFIG. 7.

FIG. 12 illustrates three feeler pins of the present invention eclipsinga sheet of light.

FIGS. 13A-B illustrate the shapes of portions of the sheet of lightwhich are eclipsed by the feeler pin of FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 illustrates a coordinate system containing X-, Y-, and Z-axes. Anobject 17 positioned in the coordinate system possesses six degrees offreedom. The object 17 can slide parallel to itself (that is, translate)along any of the three axes, thus illustrating three degrees of freedom.(Positive and negative translations, that is translations in oppositedirections but along the same axis, are herein considered to provide asingle degree of freedom). Further, the object 17 can rotate about anyof the three axes, that is, in the direction shown by arrows 18, 19, and20, thus exhibiting three additional degrees of freedom.

FIG. 4 illustrates part of one form of the present invention used tofixture the object 17 and eliminate all degrees of freedom. The object17 is supported by three point supports 22A-C which define a plane,namely, the X-Y plane. The point supports 22A-C can resemble pencilpoints in size and shape, but are constructed of a more durablematerial, such as steel.

The object 17 can slide along the X-axis to occupy the phantom outline24; it can similarly slide along the Y-axis (sliding not shown); and itcan rotate (not shown) about the Z-axis, all despite being in contactwith the three point supports 22A-C. That is, three degrees of freedomremain. These three degrees are the following: translation in theX-direction, translation in the Y-direction, and rotation about theZ-axis. (Rotation about an axis is said to occur either when an objectrotates about that axis, or about a line parallel to the axis.)

FIG. 5 shows the addition of two further point supports 25A and 25B tothe embodiment of FIG. 4. (Point support 26 is shown but not yet to beconsidered). These two point supports 25A and 25B prevent respectivetranslations along the Y- and X-axes, but, as FIG. 6 shows (FIG. 6 beinga top view of FIG. 5), the object 17 can still rotate about the Z-axisas shown by phantom outline 28 and still contact all five point supports22A-C and 25A-B. One degree of freedom still remains. However, oneadditional point support, namely point support 26 in FIG. 5, willeliminate the remaining degree of freedom, provided that the object 17is kept in contact with all six point supports 22A-C, 25A-B, and 26.

The six support points can be viewed as simultaneously providing threepoint supports such as 22A, 25A, and 25B to eliminate the threetranslational degrees of freedom and, in addition, three pairs of pointsupports, such as pairs 22A and C, 22A and B, and 25A and 26, toeliminate the three rotational degrees of freedom.

Viewed another way, the three point supports 22A-C define a plane (theX-Y plane) at the points at which they contact the object 17 (i.e., atfirst, second, and third contact points). The two point supports 25A and26 define a line 27 by their own two contact points (i.e., fourth andfifth contact points) on the object 17. This line must be noncoplanarwith, and nonperpendicular to, the plane defined by the first, secondand third contact points. Otherwise, a situation analogous to that shownin FIG. 6 can occur, since the point support 26 would lie directlybeneath point support 25A if the line 27 were perpendicular to the X-Yplane. the last point support, 25B, must provide a sixth contact pointwhich fulfills three conditions. One, the sixth contact point must benoncoplanar with the first, second, and third contact points because, ifcoplanar, the sixth contact point adds nothing to their functions. Two,the sixth contact point must be noncolinear with the fourth and fifthcontact point for a similar reason. Three, the sixth contact point must,together with the fourth and fifth contact points, define a plane whichis nonperpendicular with the plane of the first, second, and thirdcontact points: if perpendicular, with reference to FIG. 5, the sixthcontact point would lie in the X-Z plane and translation of the object17 in the negative X-direction would not be prevented.

Viewed still another way, the three point supports 22A-C define a firstplane by their contact points and three point supports 25A-B and 26define a second plane. (The requirement that point support 25B benoncolinear with point supports 25A and 26 results in the three pointsupports 25A-B and 26 defining a plane.) The point supports 25A-B and 26must be noncoplanar with the first plane and the first and second planesmust be nonperpendicular (i.e., nonnormal). Of course, all of the pointsupports 22A-C, 25A-B and 26 must be supported by some means such as abracket (not shown). Thus, one embodiment of the present invention hasbeen described wherein an object 17 is supported in space by six pointsupports.

A second embodiment of the present invention is shown in FIG. 7 whereina fixturing device 33 is shown having base 36 which supports aright-angled bracket 38. Supported by the bracket 38 is a pair ofcylinders 40A and B which fit tightly into holes drilled in a block 43which is fastened to the bracket 38. The cylinders are parallel. Asshown in FIG. 8, portions 45A and B (herein termed edges) of thecylinders 40A and B, protrude beyond surface 48 of the block 43 toprovide contact edges for a turbine engine blade, shown as outline 51 inFIG. 7. The cylinders 40A and B are fitted tightly into the holes toreduce bending or other deflection when the blade 51 is in contact withthem.

The bracket 38 further supports a pair of nonparallel cylinders 52A andB, which are in this case perpendicular, and are similarly fittedtightly into, and protrude from, cylindrical holes contained in blocks54A and B. Two spheres 56A and B are fastened respectively to thebracket 38 and to the block 43. The spheres 56A and B as well as thecylinders 40A-B and 52A-B are preferably composed of a hard metal havinga Rockwell hardness preferably in excess of 55 and commonly designatedas machine steel. The gas turbine engine blade 51 is positioned so thatportions of the blade 51 simultaneously contact all four cylinders 40A-Band 52A-B and both spheres 56A and B. That is, these cylinders andspheres provide six point supports analogous to the six point supportsdiscussed in connection with FIGS. 4-6.

Fixturing of the blade will now be described in greater detail. Toestablish reference directions, a side view of a blade 51 is shown inFIG. 9 affixed to a turbine engine rotor 53 which rotates in thedirection shown by arrow 56 about a center 59. The blade 51 is generallyaligned in a radial plane (shown as a line 61). The blade has a lowpressure side 63 and a high pressure side 65.

The blade is also shown in FIG. 10, which is a view of FIG. 7 takenalong lines 10--10. The blade 51 comprises a root portion 67 havingdovetail pins 68 which mate with dovetail slots 71 in FIG. 9. The bladefurther comprises a platform 73, and an airfoil portion 75 having aleading edge 77, a high pressure side 65, and a low pressure side 63.

The cylinders 40A and 40B contact dovetail pin 68 on the low pressureside 63 of the blade 51. Sphere 56A contacts the radially inner side ofthe platform 73 (that is, in FIG. 9, the side toward the center 59 ofrotation of the rotor). Sphere 56B contacts an upstream portion 81 ofthe root portion 67. Cylinder 52A contacts the leading edge 77 of theairfoil portion 75, and cylinder 52B contacts the low pressure side 63of the airfoil portion 75. The blade 51 is simultaneously urged intocontact with all four cylinders 40A and B, 52A and B, and two spheres56A and B by a clamp (not shown) which exerts a single force at a singlepoint on the blade 51 and generally in the direction of an arrow 82shown in FIG. 7. It is noted that the application of more than one forcecan cause a moment to occur about one or more point supports. This isnot desired because it may cause the blade 51 to lose contact with otherof the point supports.

The cylinders 40A-B and 52A-B, and the spheres 56A and B are hereintermed point supports even though, structurally, they do not havepoints. They are so termed because they contact the respective bladeportions at regions, such as region 85 in FIG. 10, which are viewed astheoretically discrete points. These discrete points are tangent pointsof the spheres and cylinders. Cylinders 40A-B and 52A-B could bereplaced by spheres (not shown), but cylinders, in providing elongated,linear edges for the blade to contact, accommodate a greater margin forblade-to-blade variations in size than do spheres.

As shown in FIG. 7, a movable support, such as a pivot plate 89 whichpivots about an axis 91, is attached to the bracket 38 and supportsthree feeler pins 93A-C. The three feeler pins 93A-C are free to slidein three respective holes 94A-C contained in the pivot plate 89. Asshown in FIG. 11, the feeler pin 93A (all feeler pins 93A-C arepreferably identical) has a conical portion 98 having a geometric apex101. The conical portion 98 is preferably a portion of a right circularcone. However, in order to reduce damage otherwise inflicted upon theapex 101 through normal wear and tear, a hard steel sphere 102 isfastened to the conical portions 98 at the apex 101, but fastened sothat the center 103 of the sphere 102 is positioned on an axis 106 ofeach cone and a tangent point of each sphere contacts the apex 101.Thus, each cone has a blunted point, but the apex 101 is still definedas a tangent point of the sphere 102. Further, each cone has a apexangle 108 equal to twice the arc tangent of the number one-half (thatis, the apex angle 108 is an angle of 53.130 . . . degrees). As shown inFIG. 11, this provides the feature that, at any given altitude of thecone (such as altitude 110), the altitude 110 is equal to the width 112.This serves a function which will now be described.

To measure the height of a blade 51 in FIG. 7, the pivot plate 89 isrotated to a predetermined position shown by phantom outline 115. Thepredetermined position is determined by the position of the axis 91 andby a suitable stop 117. As shown in FIG. 12, wherein the pivot plate 89is removed for clarity, feeler pins 93A-C are brought into contact withthe tip 120 of the blade 51 and are firmly held in contact with the tip120 of the blade 51 by springs 122A-C shown in FIG. 7 which urge thefeeler pins in the direction indicated by arrow 125 in FIG. 12. Thus,the apex points 101 are brought into contact with the tip 120, and theaxes 106A-C of the feeler pins 93A-C are positioned generally parallelto the radial plane 61 shown in FIG. 9.

A sheet of light 128 is projected across the feeler pins 93A-C andperpendicular to the axes 106A-C. While termed a sheet of light, thesheet 128 actually comprises a cylindrical laser beam (not shown)scanned in the direction of arrows 130A and B so as to cover a swathresembling a sheet. One apparatus which is commonly used to provide sucha sheet of light is a scanning laser dimensioning system such as oneavailable as Telemetric Model 121 available from Zygo Corp.,Middlefield, Ct. Each feeler pin 93A-C eclipses a respective eclipsedregion 135A-C of the sheet 128 and the width of each eclipsed region ismeasured by photosensitive equipment known in the art. One such type ofequipment is included in the Telemetric Model 121 just mentioned. Aphotodiode array 148 known in the art and shown in FIG. 12 can also beused to produce signals indicative of the width of the eclipsed region.

Since the width 112 in FIG. 11 equals the altitude 110 of each feelerpin 93A-C, measurement of the width of each eclipsed region 135A-Cprovides an indication of the distance 137 in FIG. 12 between the tip120 and the sheet of light 128. Thus, the height of the blade 51 withrespect to the sheet of light 128 is ascertained. Since the sheet 128 isconsidered to be a reference fixed in space, the distance from the tip120 to a reference is thus ascertained. Further, since three feeler pins93A-C contact the blade tip 120, three points thereon are measured indistance from the sheet of light 128. These three points define a planeof the surface of the tip 120 and the measurment of the displacement ofentire plane of the surface of the tip 120 after welding will now bediscussed.

This embodiment can be utilized to inspect the quality of the weldmentwhen a tip cap 139 in FIG. 7 is diffusion-bonded to the tip 120 of theblade 51. First, the blade 51 is positioned in the fixturing device 33described above and the tip cap 139 is positioned between the feelerpins 93A-C and the blade 51. This positioning simulates the actualposition which the tip cap 139 will assume when bonded to the blade 51.The feeler pins are brought into contact with the tip cap 139 and thewidth of the eclipsed region 135A-C in FIG. 12 of each is measured (thetip cap 139 is not shown in FIG. 12 positioned atop the blade tip 120).The blade is removed, a layer of metal welding material is placedbetween the tip cap 139 and the tip 120, and the blade assembly isheated in a furnace. In the case of a layer of weld material which is 3mils thick (that is, three thousandths of an inch), it is commonlyassured that the final weld, if properly executed, will separate the tipcap 139 and the blade tip 120 by a distance of 1 mil. After diffusionbonding, the blade 51, together with the bonded tip 139 are againmeasured in height. The second measurement will, if resulting from asuitable diffusion bond, be one mil less than the pre-bond measurement.That is, each apex point 101 will be displaced one mil in the directionof arrow 136 in FIG. 12 and the sheet of light 128 will strike eachconical region 98 in FIG. 11 at a region of lesser width 112 thanbefore. Deviation of the displacements from a predetermined number suchas one mil indicates a possibly defective diffusion bond. Nonuniformdisplacement of one apex point 101 with respect to another indicatesthat the plane of the surface of the tip 120 was displacednonuniformally and thus indicates the possible existence of a partialfault in the weldment.

It is recognized that the sheet of light 128 is not infinitesimallythin, but will possess some finite thickness. Thus, the portion 135A inFIG. 12 eclipsed by the feeler pin 93A of FIG. 11 will have the generalshape shown in FIGS. 13A-B. When the feeler pin 93A is displaced in thedirection of arrow 136 in FIG. 12, a new eclipsed portion 135AA in FIG.13B will be generated as shown by sheet portions 128A. The eclipsedportion 135AA will be narrower than eclipsed portion 135A: width 150¹ isless than width 150.

The fact that the eclipsed portion 135A possesses a nonuniform width (asshown by widths 150 and 151) presents no significant problem. The changein width of any one of the widths, such as width 150, is sought to bemeasured. Measuring such a change is known in the art.

Further, in the discussion relating to FIG. 12, it was stated that ameasurement is undertaken between the sheet of light 128 as a referenceand the tip 120 of the blade 51. Of course, this assumes that the sheetof light 128, even though having a finite thickness 160 as shown in FIG.13A, can be viewed as defining a discrete reference position, such aslower edge 156. This is a matter of calibration. Such calibration isknown in the art and is not part of the present invention.

Apparatus have been described having one embodiment for supporting anobject such as a gas turbine engine blade accurately and repeatably inspace. The blade is supported by six point supports of which four aresupplied by rigidly positioned cylinders and two are supplied by rigidlypositioned spheres. Feeler pins having blunted conical regions aresupported on a movable support and the apex of each conical tip isbrought into contact with the tip of the blade. A sheet of light isshone onto each conical region, and the relative altitude, which is afunction of the radial height of the blade, at each contact point of theapex is determined by the width of the light eclipsed. In contacting thetip at three points, a triangle is defined which, is as well known,defines a plane. Thus, the location of the plane of the tip is defined,and displacement of this plane following diffusion bonding is measured.

Numerous substitutions and modifications can be undertaken withoutdeparting from the invention as defined by the spirit and scope of thefollowing claims.

What is desired to be secured by Letters Patent of the United States isthe following.

I claim:
 1. Apparatus for supporting a gas turbine engine blade whichhas a root having an upstream portion and a dovetail portion; a platformhaving a radially inner side; and an airfoil having a tip, a leadingedge, and a low pressure side, comprising:(a) a bracket; (b) a sphericalfirst support positioned at a first predetermined position with respectto the bracket for supporting the upstream portion of the root; (c) aspherical second support positioned at a second predetermined positionwith respect to the bracket for supporting the platform; (d) cylindricalthird and fourth supports positioned at respective third and fourthpredetermined positions with respect to the bracket for supporting thedovetail portion; (e) a cylindrical fifth support positioned at a fifthpredetermined position with respect to the bracket for supporting thelow pressure side; (f) a cylindrical sixth support positioned at a sixthpredetermined position with respect to the bracket for supporting theleading edge; and (g) measuring means for measuring the distance betweena predetermined point on the blade and a predetermined reference. 2.Apparatus according to claim 1 in which the predetermined referencecomprises a sheet of light and the measuring means comprises a movablefeeler pin which contacts the blade and eclipses the sheet of light inan amount dependent upon the distance between the sheet of light and thepredetermined point on the blade.
 3. Apparatus according to claim 2 inwhich the feeler pin comprises a conical portion having a vertex anglesubstantially equal to the arc tangent of the number one-half and whichconical portion eclipses the sheet of light.
 4. Apparatus for obtaininginformation concerning the uniformity of a diffusion bond of a tip capof a blade used in a gas turbine engine and having a root, a platform,and an airfoil having a leading edge and a low pressure side,comprising:(a) a bracket; (b) first and second point supports forrespectively supporting the root at a first point and the platform at asecond point; (c) third and fourth point supports for respectivelysupporting the root at third and fourth points; (d) fifth and sixthpoint supports for respectively supporting the low pressure side at afifth point and the leading edge at a sixth point, the six pointsupports being located at predetermined positions with respect to thebracket; (e) clamping means for pushing the blade into simultaneouscontact with all six supports of paragraphs (b)-(d); (f) three feelerpins comprising three right circular cones having three respectiveparallel central axes, each cone blunted at the vertex, each feeler pinfor contacting the tip of the blade when the blade is clamped by theclamp of paragraph (e) and for extending in the radial direction of theblade during contact; (g) illumination means for projecting a sheet oflight perpendicular to the central axes of paragraph (f) for eclipsingof the sheet of light by the feeler pins; and (h) means for sensing thewidths of the eclipsed sheet of light of paragraph (g) and for producinga signal indicative thereof.